1. Field of the Invention
The present invention relates to a device for detecting the optimum anode load impedance of a tube transmitter in a high frequency transmission chain.
2. Description of the Prior Art
A tube transmitter cannot be directly loaded by a transmission antenna whose impedance is much too low; there is generally used between the anode and the antenna a matching cell using variable inductances and capacities which must be tuned in accordance with the working frequency and the impedance of the antenna.
This adjustment may be controlled in numerous ways which all boil down to ensuring that the load impedance of the tube is optimum, which corresponds to a maximum efficiency of the tube and maximum power radiated by the antenna. They are based on different principles also involving different technologies.
A first method consists in measuring the HF current (I) and the HF voltage (V) at the output of the tube. The ratio of these two measurements gives directly the desired parameter Za but as soon as the power becomes high it is difficult to obtain the current information by toric coupling.
A second method consists in measuring the mean current consumed and in effecting a comparison with the HF power transmitted by the transmitter. The technology is simple but the accuracy is poor and the adjusting algorithm is complex.
A third method consists in substituting for the measurement of the V/I ratio, measurements (easier to make) of the HF voltaes existing respectively at the input electrode of the tube (V.sub.E) and at the output electrode (V.sub.S). These two elements are connected by the relationship: EQU V.sub.S =S Za V.sub.E
where Za is the desired anode load impedance and S a constant characteristic of the tube, called slope. Matching consists then in detecting, during adjustment of the anode load impedance, the precise moment when the phase of the V.sub.S /V.sub.E ratio becomes equal to 180.degree. (grounded cathode) and when the modulus of this ratio, equal to the product of the slope of the tube multiplied by its load resistance, becomes equal to a previously defined value. It is not necessary to operate directly on the ratio V.sub.S /V.sub.E, but it is in fact sufficient to effect a modulus and phase comparison of the signals V.sub.S and V.sub.E.
It is known, for more easily obtaining an adjustment convergence, to assign the modulus and phase adjustment parameters respectively to a single one of the variable elements forming the matching cell.
However, in the devices for detecting the optimum anode load impedance known up to now and operating in accordance with the third above-mentioned method, the phase matching detection, namely the measurement for determining the end of the phase adjustment, is dependent on the adjustment operations carried out on the modulus, and vice versa, which does not allow full benefit to be drawn from the anticipated results by distributing the adjustment parameters between the modulus and the phase.
The present invention has as its aim to avoid this drawback.
Moreover, the device for detecting the optimum anode load impedance in accordance with the invention allows, with respect to known devices, the accuracy of adjustment to be increased and the adjustment algorithm to be simplified, by seeking not extrema (variables depending on the levels) but zeros, which allows a great adjustment dynamic to be obtained depending on the power brought into play, but especially the adjustments to be made independent of this power, with immediate determination of the direction of variation of the adjustment elements.